The present invention relates to polymer compositions particularly suitable for agricultural purposes. More specifically, the invention relates to polymer films useful as covering materials for growing plants or crops.
It is well known by the actual state of the art the growing of plants under tunnels and green houses with plastic film opaque to long wave infrared radiation, particularly in the range 7-14 micrometers. This range is specifically transparent to the dry atmosphere. Those techniques have resulted in the production of earlier and more abundant crops.
The beneficial effects which these films have on the crops become even more pronounced as the film is made more opaque to the effects of the infra-red rays, within the above mentioned range of wavelengths yet at the same time, more transparent to the visible light necessary for photosynthesis.
These films, which are internationally known as "thermals", also reduce the risk of frost damage to the crops when heating is not available in the hothouse. Likewise, when heating is available, the films reduce the energy consumption. Both of these effects are attributable to the reduction of heat loss by radiation.
Most commercially available plastic films which are transparent to visible light are in some measure opaque to the infra-red radiation previously mentioned. The one most used worldwide for this application is polyethylene film, due to other important advantages both technical and economic. However, it is very transparent to the infra-red radiation ranging between 7 to 14 micrometers.
Previous attempts to alleviate the problems associated with the use of polyethylene film as a protector for crops have involved the addition of small quantities of "thermal additives", preferably of the mineral type such as: natural silicates (talc, kaolin, etc.), silica, calcium carbonate, barium sulphate, aluminum hydrate, and metallic hydroxysulphates, etc.
The following patents which disclose the aforementioned are:
French Pat. Nos. 1,574,088 of Apr. 16, 1968 and its continuation, 2,213,954 of Jan. 15, 1973; Japanese Pat. No. 7,213,853 of May 15, 1968; Belgian Pat. No. 845,925 of Sept. 9, 1975; Japanese Pat. Nos. 5-1074-846 of Dec. 18, 1974 and 5-0088-147 of Nov. 8, 1973; and U.S. Pat. Nos. 4,134,875 of Mar. 17, 1978 and 4,170,087 of Oct. 9, 1979; as well as the article "Thermal Films for Hothouses", Seventh International Congress on Plastics in Agriculture by Graciano Jorge Tapia, Lisbon, October 1980.
The addition of the above-listed products to the polyethylene presents some disadvantages. For example, when used alone, most of them have to be added in large quantities in order to achieve good opacity to the infra-red radiation falling between 7 and 14 micrometers. However, these large quantities act as a filler on the polyolefin base, decreasing the mechanical properties and, more importantly, the overall transmission of the visible light.
To alleviate the problems associated with the use of a single additive, techniques are currently being studied whereby two or more of them are mixed together. However, such mixing greatly complicates the manufacturing process and reduces the ability of the polyethylene film to transmit the visible light necessary for photosynthesis.
Additionally, there are problems peculiar to many of the additives. For example, when the silica and silicates are exposed to the open air, they act as degrading agents on the polyethylene, thus reducing its life. The aluminum hydrate decomposes above the temperature of 180.degree. C., releasing moisture, which enormously complicates the manufacturing processes, both of the compound and later of the film.